Information about the mechanical properties of bones is useful in many areas of orthopedic medicine. One area is in diagnosing and treating osteoporosis, a calcium-depletion disease prevalent in post-menopausal women. Another is in assessing the degree of healing which has occurred in a fractured bone. Loss of bone strength and stiffness by disuse is also a concern, where a patient has undergone long periods of immobilization. It is also desirable to monitor changes in bone mechanical properties during bone-related therapies, such as calcitonin treatment of osteoporosis, for purposes of evaluating and improving therapies.
One method for assessing the mechanical properties of bones which has been used clinically is photon absorptiometry, which provides a direct measure of bone mineral content (density). A single-beam method is used to monitor arm, heel, and lower bones of the leg, and a dual-beam technique, to monitor spine and femur. Although the method provides a good measure of bone mineral density, it does not reveal the condition of the bone matrix itself, i.e., the collagen-containing matrix which gives the bone its bending stiffness and load characteristics. For some conditions, like osteoporosis, bone mineral content appears to be a good indicator of bone health (Orne), and therefore photon absorption measurements are generally useful for diagnosing the disease state and monitoring therapy. However, for other conditions, such as fracture healing, bone mineral content may correlate only weakly with bone healing, and in these areas, the technique is of limited value.
Other limitations of single- and dual-beam photon absorptiometry include patient exposure to ionizing radiation, relatively long scan times (20 minutes or more) and complex and relatively expensive equipment.
Another bone-analysis approach which has been proposed heretofore is based on the response of bones to mechanical vibration. Attempts to use the mechanical measurement of bone resonance frequency for the evaluation of fracture healing and osteoporosis have been reported (Campbell). In theory, the method is capable of determining bone stiffness from the force and displacement measured during mechanical stimulation. This approach has been severely limited heretofore by soft tissue effects which tend to mask force and displacement values related to bone only. This problem may be partially solved by small vibrators which are pressed tightly against the tissue region of interest, in effect, establishing a more direct contact between the probe and the bone. However, mechanical stimulation with this arrangement tends to be painful, and in any case, does not totally eliminate soft tissue effects. An alternative, purely static approach which has been proposed (Stein) has severe problems of reproducibility. Another approach uses impact response (Wong). However, results are difficult to interpret and appear to be strongly dependent on soft tissue effects.
The inventor has previously proposed various mechanical response systems in which soft tissue effects can be reduced by (a) estimating soft tissue effects at higher frequency vibrations, where bone response is minimal, and (b) subtracting out soft-tissue effects from low-frequency measurements, as discussed in Petersen, 1975, 1977; Steele, 1978, 1984, 1985; and Young 1982, 1984. One such system, developed by the inventor and coworkers, has been tested on several hundred patients. Although the method has been applied with some success to many subjects, it has serious shortcomings where the subject is obese or shows heavy musculature. i.e.. where soft tissue effects are large. Also calculation times lend to be quite long, on the order of at least several minutes.